Influences of Wind and Snow on Northern Tree-Line Environments at Churchill, Manitoba, Canada

The influences of wind and snow distribution at the tree line near Churchill, Manitoba, were studied. Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experiments with white spruce branchlets. For white spruce, the abrasion zone was evidently within 80 cm above the snow surface, and needle removal by abrasion was strongly influenced by branch age. Removal by abrasion of similar-aged needles was highest in new branches and declined with branch age up to 7 or 8 years, and then In forest-tundra environments the matted forms of trees were snow covered early in winter and received little abrasion. Sparsely needled islands of trees were largely covered with snow at the base. More densely needled trees and some trees within woodlands were less exposed to abrasion. The blockage of air flow resulted in high-speed jets, which cleared out snow within a "horseshoe-shaped vortex" around the base of the tree. Both in much of the woodlands and in the open forest, snow abrasion was minimal or non-existent and snow load on the branches (qali) becomes the prevalent stress. During winter, a large proportion of the snowfall was initially blown off the exposed surfaces of Hudson Bay and the coastal tundra regions, into the woodlands, and eventually across the tree line and into the open forest. As the woodlands filled up with snow in mid-winter, the rate of snow accumulation in the forest increased from double to triple the snowfall. Variations in the rate of accumulation occurred with wind speed and direction. During May, snowmelt began on exposed tundra first and usually ran off the frozen surface. Snowmelt occurred about three weeks later in the open forest and was characterized there by less runoff, as the water more readily permeated the thawing peat. The late snow beds are characteristic of the forest-tundra woodlands and were usually gone by mid-July. The woodlands were snow free for 1.5-2 months during the year, while the open forest was snow free for about 3 months and the tundra was largely snow free for 6 months or more.